Vitreous bonded alumina abrasive grain grinding wheels have enjoyed large popularity and use for many years in grinding operations for a large variety of metal workpieces, particularly iron and steel workpieces. The principle alumina abrasive grain used in such grinding wheels has been and continues to be fused alumina abrasive grain. Such past and continued use of fused alumina grain has in large measure been based on economic considerations, even though less than desirable grinding performance was often achieved. Fused alumina grain is low cost. This less than desirable performance is often manifested not only in the reduced quality of the finished parts, but also in other considerations such as higher power consumption, heat generation, friction, reduced production rates, higher costs and low G-ratio.
The last factor (i.e., G-ratio) is the ratio of the volume of metal removed from the workpiece to the volume of wheel lost during the grinding operation. Thus it is the volume of metal removed per unit volume of wheel wear. A low G-ratio indicates a small amount of metal removed from the workpiece per unit volume of wear or loss of the grinding wheel and therefore indicates very little grinding is taking place and/or high wear of the grinding wheel. The high wear of the grinding wheel results in a short useful life of the wheel, increased wheel usage and increased wheel cost in the grinding operation. A high G-ratio shows a large amount of metal removed from the workpiece per unit volume of wheel wear or loss. Such high G-ratio indicates longer useful wheel life, lower wheel usage, greater cost effectiveness, higher production rates and more efficient grinding. Reduced power consumption, lower friction and reduced heat often accompany a high G-ratio grinding operation. As the G-ratio increases there often can be accompanying increases in wheel speed, infeed rates and workpiece rotation speed. These increases lead to higher production rates. Thus, higher G-ratio is desirable and sought after in the development and improvement of grinding wheels, especially in wheels employing costly abrasive grain (e.g., super abrasive grain). Increasing the G-ratio of grinding wheels containing more expensive grain makes such wheels more cost competitive with lower G-ratio wheels made with low cost abrasive grain and increases their cost effectiveness in precision, critical and difficult grinding operations (e.g., grinding tool steel and exotic alloys).
More recently an improved alumina abrasive grain was developed using a sol-gel and sintering process rather than the fusion process for producing alumina abrasive grains. Such newer abrasive grains proved to be more costly than the fused alumina grain. However, grinding wheels made with the sintered sol-gel alumina abrasive exhibited improved grinding performance over wheels made with fused alumina grain. This improved performance was obtained at a higher grinding wheel cost, particularly when such grain is used in vitreous bonded grinding wheels. Thus, sintered sol-gel alumina abrasive grain have not achieved the popularity and wide spread use found for fused alumina abrasive grains. Increasing the grinding performance, especially the G-ratio, of grinding wheels produced with sol-gel alumina abrasive grain , particularly vitreous bonded grinding wheels made with such abrasive grain, would make them highly competitive with wheels produced with fused alumina abrasive grain in a wide range of grinding applications.
The sintered sol-gel alumina abrasive grains can be produced by a process comprising the steps of preparing a dispersion of an alumina monohydrate to which a modifier may be added, gelling the dispersion, drying the gelled dispersion, crushing the dried gelled dispersion to form particles, calcining the particles, and firing the particles to form abrasive grains. Various adaptations and modification of this basic process have been developed and disclosed since the process was first discovered and disclosed to the art. The firing step is carried out to sinter the grains at temperatures below the fusion temperature of aluminum oxide. The sol-gel process of making alumina abrasive grains is more fully described in U.S. Pat. No. 4,314,827 to Leitheiser et al, which disclosure is incorporated herein by reference.
The first sol-gel alumina abrasive grains were disclosed in U.S. Pat. Nos. 4,314,827 and 4,518,397 to Leitheiser et al. These patents teach a non-fused abrasive grain comprising alpha alumina and at least one modifier selected from the group consisting of oxides of zirconium, hafnium, cobalt, nickel, zinc, and magnesium, and may include secondary modifiers such as samaria, titania and ceria.
In U.S. Pat. No. 4,623,364 to Cottringer et al disclosure is made of the addition of an alpha alumina seed material to the non-alpha alumina dispersion, in the sol-gel process, to enhance the transformation to alpha alumina during sintering.
Disclosure is made in U.S. Pat. No. 4,744,802 to Schwabel of a sol-gel process wherein iron oxide or an iron oxide precursor is added to the alumina dispersion for producing a durable alpha alumina abrasive grain.
A ceramic abrasive grain comprising alpha alumina and at least about 0.5% yttria modifier is disclosed in U.S. Pat. No. 4,770,671 to Monroe et al. The abrasive grain is especially efficient for abrading stainless steel and exotic alloys.
The impregnation of a metal oxide modifier into a porous sol-gel process alumina body is taught in European Patent Application, Number EP 293,163A, published November 1988, in a process comprising preparing a dispersion of alpha alumina monohydrate, gelling the dispersion, drying the gelled dispersion, crushing the dried dispersion to form particles, calcining the particles, impregnating the particles with a dispersion of a precursor of the metal oxide, drying the impregnated particles, calcining the dried particles and firing the dried particles to form abrasive grains. The modifier can be an oxide of magnesium, zirconium, hafnium, nickel, cobalt, zinc, titanium, yttrium, or a rare earth metal.
Sol-gel process ceramic abrasive grain comprising alpha alumina and at least 0.5% of a rare earth metal oxide is taught in U.S. Pat. No. 4,881,951 to Wood et al. The rare earth metal may be selected from praseodymium, samarium, ytterbium, neodymium, lanthanum, gadolinium, cerium, dysprosium, erbium and mixtures thereof. Impregnation of the rare earth modifier as in EP 293,163A into the unfired alumina particle is also taught.
The modification of sol-gel alumina abrasive grain with from 0.01 to 2% ceria is taught in U.S. Pat. No. 5,053,369.
There is disclosed in The Society of Manufacturing Engineers Paper EM 90-360, entitled "Use of Sintered Ceramic Aluminum Oxides in Vitrified Bonded Wheels", published Sep. 12, 1990, the grinding performance of vitrified bonded abrasive wheels employing ceramic alumina-based abrasive grains.
The use of sol-gel abrasive grain in bonded abrasive articles is known in the art. However, no recognition or significance is given or suggested in the art that the G-ratio of a vitreous bonded sol-gel alumina abrasive grain grinding wheel or other bonded abrasive article is particularly and significantly enhanced by the presence therein of sol-gel alumina abrasive grain having a high density, particularly a density of at least 3.85 grams per cubic centimeter (g/cc). It is known that a given abrasive grain can be produced with a range of densities by varying the pore content and composition of the grain. The greater the pore content the lower the density and conversely the lower the pore content the higher the density without composition variation. At zero pore content the maximum density is typically achieved. This maximum density can be the theoretical density of the grain. As used in this disclosure the phrases sol-gel alumina abrasive grain and sintered sol-gel alumina abrasive grain shall be used interchangeably and shall mean alpha-alumina-based abrasive grain produced by the sol-gel process.
Alumina-based abrasive grains produced by the sol-gel process are to be distinguished from alumina abrasive grains made by conventional sintering of particulate alpha-alumina and by fusion processes. The sol-gel process includes the preparation of an aqueous dispersion of alumina monohydrate to which modifiers may be added, gelling the dispersion, drying of the gel, crushing the dried gel to produce particles, calcining the dried particles and firing the calcined particles. These process steps are not included in the production of conventional sintered or fused alumina abrasive grain. Owing to the differences in the processes for producing sol-gel, sintered and fused alumina abrasive grains there are found differences in grain structure, grain properties and grain performance in grinding wheels and other abrasive articles, particularly vitreous bonded grinding wheels and other bonded abrasive articles (e.g., honing stones). While sol-gel abrasive grains have been recognized in the art to provide superior grinding performance over fused alumina abrasive grains in vitreous bonded grinding wheels, there has been no recognition that high density sol-gel alumina grains would produce an unexpectedly high performance improvement in vitreous bonded abrasive products.